Basion-axial interval (BAI)

What it is

The basion-axial interval (BAI) is a measurement assessing the horizontal (front-to-back) alignment between the basion (front edge of the foramen magnum) and the axis (second cervical vertebra). It is used, often together with the basion-dens interval, to evaluate craniocervical alignment and stability.

Understanding Basion-axial interval (BAI)

The BAI reflects how the skull base is positioned horizontally relative to the upper spine. An abnormal BAI can suggest abnormal alignment or instability at the craniocervical junction — for example, abnormal front-to-back displacement between the skull and spine. In Chiari evaluation, the BAI is one of the craniometric measurements used to detect craniocervical instability or abnormal alignment, which is important because such findings can change treatment (sometimes requiring fusion). The BAI and BDI together (sometimes combined in rules assessing the skull-spine relationship) provide a fuller assessment of craniocervical alignment than either alone. Understanding the basion-axial interval clarifies another measurement used to assess craniocervical alignment and stability in Chiari.

Why it matters

Understanding the basion-axial interval helps families see how the horizontal alignment of the skull over the spine is measured to assess for instability — relevant in Chiari evaluation and decisions about treatment.